Processes for the biological conversion of bisulphide into elemental sulphur by means of sulphide-oxidising bacteria are well-known in the art. Typically in such processes, a liquid alkaline absorbent that has been used for absorption of hydrogen sulphide from a sour gas stream and that comprises absorbed hydrogen sulphide, predominantly in the form of bisulphide but typically also to some extent sulphide, polysulphide and/or dissolved hydrogen sulphide, is contacted with sulphide-oxidising bacteria for conversion of dissolved sulphides (mainly as bisulphide) into elemental sulphur. Bisulphide in other streams, for example in a spent caustic stream, may also be converted into elemental sulphur in such biological conversion processes.
Processes comprising a sour gas absorption stage followed by biological oxidation of the dissolved sulphides are for example disclosed in WO92/10270, WO94/29227, WO98/57731, US2008/0190844 and WO2005/092788.
In the prior art processes, the bisulphide-containing alkaline absorbent obtained by sour gas absorption is contacted with sulphide-oxidising bacteria in an aerobic bioreactor in order to convert the bisulphide into elemental sulphur and to obtain regenerated absorbent that is recycled to the absorption stage. Elemental sulphur is recovered from part of the regenerated absorbent.
In the process of US2008/190844 a process is described in which the biomass and the aqueous solution as present in the effluent of a so-called aerobic Sulphur Biological Reactor are separated with the aid of a cationic coagulating agent. The biomass may be recycled to the reactor and the aqueous solution may be recycled to the sour gas absorber or scrubber. This aqueous solution, which does not comprise any biomass, may be processed in an Aerobic Biological Reactor.
In the process as for example disclosed in WO94/29227, dissolved sulphides are oxidised with sulphide-oxidising bacteria. It is mentioned in WO94/29227 that the sulphide volume load to the bioreactor is preferably below 1000 mg/l·h, more preferably below 200 mg/l·h, to avoid an excessively high effluent sulphide concentration. In biological sulphide oxidation processes, the biological oxidation to higher oxidised sulphur compounds such as sulphate or the chemical oxidation to thiosulphate is preferably prevented. It is mentioned in WO94/29227 that the oxidation to higher oxidised sulphur compounds can be substantially reduced by adjusting the oxygen supply to the bioreactor. A preferred range of 0.5 to 1.5 moles of oxygen per mole of sulphide is mentioned.
In practice it has, however, been found that even in situations where the amount of oxygen supplied to the bioreactor has been controlled within the range of 0.5 to 1.5 moles of oxygen per mole of sulphide, undesirably high amounts of sulphate and thiosulphate can be formed. There is thus a need in the art for improved prevention of (thio)sulphate formation.